Approximately sixty-five percent of all the oil discovered remains trapped underground in reservoirs following primary production (natural reservoir pressure) and secondary production (water or gas flood). Microbial enhanced oil recovery (“MEOR”) holds considerable promise for recovering a significant proportion of trapped global oil reserves.
Conventional MEOR is an empirical process whereby inexpensive nutrients are pumped into an oil reservoir to stimulate growth of indigenous and dormant microorganisms. In theory, the rejuvenated microbial community produces environmentally friendly biometabolites such as gases, acids, solvents, and surfactants that release trapped oil and/or biomass and polymers that plug water channels thereby diverting subsequent water or gas floods into oil bearing zones.
Conventional MEOR has been employed for decades and has been moderately successful but, frequently, the results have been disappointing. A typical MEOR approach is to pump molasses or agricultural fertilizer into a watered-out reservoir and hope for the best. This hit-or-miss approach is not based on scientific principles and any positive, negative, or damaging results remain unexplained. In some cases, undesirable bio-metabolites such as hydrogen sulfide have caused irreversible reservoir damage, equipment corrosion, and health threats.
There are many applications of MEOR, but none of them include prior metabolic characterization of microbial communities that inhabit oil reservoirs. According to some culture-based and genetic evidence, microbial communities are markedly different among oil reservoirs depending on rock type, temperature, depth, and various other factors. Therefore, blindly injecting nutrients into an oil reservoir and hoping for beneficial results is an uncertain and potentially damaging process. Pumping the same nutrient into several reservoirs and expecting similar results is unscientific and unreasonable. There is no way currently to predict what bio-metabolic response, if any, can be expected in a given oil reservoir when nutrients are injected. Therefore, it would be beneficial to have a method for growing reservoir microorganisms in a controlled and scientific way.
Targeted, scientifically-based MEOR treatments could be devised for individual oil reservoirs if one knew the likely metabolic response of the microbial community to an infusion of nutrients. Then one would need to stimulate the desirable microbes and suppress the undesirable ones, for example, sulfate-reducing bacteria responsible for souring oil. To do this in a scientific fashion, one has to know what species of bacteria live in a given reservoir, what the actions of the microbial community in a given reservoir to nutrient infusions, what bioproducts they are capable of producing, and exactly what nutrients and co-factors they need to grow at optimum rates. However, most reservoir microbes die when brought to the surface in a sampler, when those microbes are exposed to air, low temperature, low pressure, and a variety of other stressors. Few, if any, indigenous microbial species survive when hoisted to the surface. Therefore, conventional laboratory culture of oil-reservoir microorganisms in Petri dishes or in flasks of liquid growth media at room temperature is not feasible.
The basic goals of MEOR are: (1) to stimulate desirable reservoir microbes, for example, those that produce useful quantities of oil-releasing or channel-plugging materials; and (2) to suppress undesirable ones, for example, sulfate-reducing bacteria responsible for souring oil, corroding pipes and equipment, and that pose a toxic hazard to workers. To do this in a scientific and predictive fashion, one must know what bio-metabolites reservoir microbes are capable of producing, and exactly what nutrients, supplements, and co-factors they need to grow and produce specific bio-metabolites at optimum rates. However, culturing reservoir microorganisms in the laboratory to elucidate microbial community metabolism is unsatisfactory because ninety-nine percent or more die when brought to the surface because of the sudden decreases in pressure and temperature and the exposure to oxygen. Therefore, it would be beneficial to have a method for growing reservoir microorganisms under anaerobic conditions and under reservoir conditions of high temperature and high pressure.